"This transaction is invalid." Error

Hi there,

Running OC 2.1.0.1 with PayPal Payments Standard.

Some customers seem to have an error when they go to PayPal to complete their purchase, which states, "This transaction is invalid. Please return to the recipient's website to complete your transaction using their regular checkout flow."

I believe it only happens to some customers and not others, as we still receive orders while others have this error.

Is there any idea to what causes this and if there is a way to fix it for the few it affects? Or at the very least have some steps to help them fix it when they have this error?

Thanks!